Non-aqueous electrolyte secondary batteries can include, for example, a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The positive electrode active material can be, for example, a lithium-containing composite oxide such as lithium cobaltate. The negative electrode active material can be, for example, a graphite material. The separator has the function of insulating the positive electrode from the negative electrode while retaining the non-aqueous electrolyte. The separator can be, for example, a microporous sheet made of polyethylene resin.
In the production process of such a non-aqueous electrolyte secondary battery, due to the friction between metal components included in machinery used as production equipment, impurities that are in the state of metal may enter, for example, the positive electrode. If such an undesirable impurity enters the positive electrode, which has a high potential, the impurity (metal) is ionized in the positive electrode and dissolved in the non-aqueous electrolyte. The ionized metal deposits on the negative electrode due to charge/discharge of the battery. When the amount of the metal deposited on the negative electrode reaches a predetermined amount or more, it grows into dendritic form. The dendritic metal impurity penetrates through the separator and reaches the positive electrode, thereby causing a short-circuit between the positive electrode and the negative electrode. Such a short-circuit results in a decrease in battery voltage and battery capacity.
It is thus desirable to minimize the amount of impurities contained in battery materials and/or the amount of impurities entering the battery in the production process.
However, the production of impurities due to the friction between metal components cannot be easily suppressed. Thus, detection methods of impurities have been proposed.
For example, Patent Document 1 proposes detecting impurities contained in a positive electrode material slurry by using a magnet. Patent Document 2 proposes detecting whether or not a thin layer of electrode materials for a lithium secondary battery contains impurities that cause magnetic disturbance by using a device that detects magnetic disturbance by magnetic impedance effect.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-358952
Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-183142